XGen patches define the areas of the geometry that generate primitives. Patches bind to the faces of the selected polygon mesh when you create the Description. Patches provide information about the geometry, such as face ID's, face size, position, and face normals, which the Description then uses to place and control the primitives. Collection (.xgen) files store the information about the patches (see Manage XGen files and directories).
In the Outliner, patches are identified by this icon: 
. In the scene view, areas bound to XGen patches display with a magenta-colored X at the center of the geometry faces. Patches do not have attributes to modify. You can change which polygon faces bind to the patch by using the Bind Face menu options.
Patch references (PRefs)
When you create a Description, Maya adds xgenPref attributes (Extra Attributes section) to the polygonShape node of the mesh bound to the patch. These attributes store initial state information about the patch and geometry. This information ensures that the density of the primitive generation remains consistent when the geometry deforms.
By default, primitive generation is based on the number of primitives per unit area (which for XGen is cm) on the mesh. If geometry deformations change the face sizes, the number of primitives generated on the mesh also change. Without the xgenPref attribute, these changes in primitive generation can cause unwanted popping between frames as the geometry deforms.
Important notes
- When you select a patch in the Outliner, attributes labeled bBox Corner appear in the Maya Attribute Editor. Do not modify these attributes.
- Do not move patches between Descriptions. Instead, import the Description to geometry with the same topology. Ensure the Include patch bindings options is on in the Import Collections or Description window.
- Do not move guides between patches.
- When you batch render a Description, you must export the patches using File > Export Patches for Batch Render. See XGen Editor File menu.